Cardiac arrhythmias are the leading cause of death in the United States. The most common cardiac arrhythmia is ventricular tachycardia (VT), i.e., very rapid and ineffectual contractions of the heart muscle. VT is the cause death of approximately 300,000 people annually.
In the United States, from 34,000 to 94,000 new patients are diagnosed annually with VT. Patients are diagnosed with VT after either (1) surviving a successful resuscitation after an aborted sudden death (currently 25-33% of sudden death cases) or (2) syncope, i.e., temporary loss of consciousness caused by insufficient cerebral circulation. The number of VT patients is expected to increase in the future, estimated to range between 61,000 and 121,000 patients annually in five years, as a result of early detection of patients at risk for sudden death by newly developed cardiac tests, advances in cardiopulmonary resuscitation, better medical management of acute myocardial infarction patients, and the demographic shift to a more aged population.
Without proper treatment most patients diagnosed with VT do not survive more than two years. The most frequent current medical treatment consists of certain antiarrhythmic drugs or implantation of an automatic implantable cardiac defibrillator (AICD). Drug treatment is associated with an average life span of 3.2 years, a 30% chance of debilitating side effects, and an average cost of approximately $88,000 per patient. In contrast, AICD implantation is associated with a life expectancy of 5.1 years, a 4% chance of fatal complications, and a cost of approximately $121,000 per patient.
In a majority of patients VT originates from a 1 to 2 mm lesion that is located close to the inner surface of the heart chamber. A treatment of VT in use since 1981 comprises a method whereby electrical pathways of the heart are mapped to locate the lesion, i.e., the "active site," and then the active site is physically ablated. In most instances the mapping and ablation are performed while the patient's chest and heart are open. Also, the mapping procedure has been carried out by sequentially moving a hand-held electrical recording probe or catheter over the heart and recording the times of arrival of electrical pulses to specific locations. These processes are long and tedious.
Attempts to destroy, i.e., ablate, the critical lesion are now quite successful, but are currently limited to a small number of patients who can survive a prolonged procedure during which they have to remain in VT for almost intolerable periods of time. The time-consuming part of the treatment is the localization, i.e., identifying the site, of the target lesion to be ablated. Another limitation preventing the widespread use of catheter ablation for VT is poor resolution of target localization, which in turn compels the physician to ablate a large area of the patient's heart. The reduction in heart function following such ablation becomes detrimental to most patients with pre-existing cardiac damage. However, once the target is correctly identified, ablation is successful in almost all patients.
An improved procedure for treatment of VT must include a faster, more efficient and accurate technique for identifying, or "mapping", the electrical activation sequence of the heart to locate the active site.
In electrophysiologic examinations, and in particular in those using invasive techniques, so-called electrical activation mapping is frequently used in combination with an x-ray transillumination. The local electrical activity is sensed at a site within a patient's heart chamber using a steerable catheter, the position of which is assessed by transillumination images in which the heart chamber is not visible. Local electrical activation time, measured as time elapsed from a common reference time event of the cardiac cycle to a fiducial point during the electrical systole, represents the local information needed to construct the activation map data point at a single location. To generate a detailed activation map of the heart, several data points are sampled. The catheter is moved to a different location within the heart chamber and the electrical activation is acquired again, the catheter is repeatedly portrayed in the transillumination images, and its location is determined. Currently catheter location is determined qualitatively or semi-qualitatively by categorizing catheter location to one of several predetermined locations. Furthermore, the transillumination method for locating the catheter does not convey information regarding the heart chamber architecture.
The present technique requires the use of a transillumination means during each of the subsequent catheter employments. This means that if the subsequent catheter locating is achieved by ionizing radiation, the patient and the physician must be subjected to a radiation exposure beyond that which would be required only for producing the basic image of the heart chamber architecture.
A catheter which can be located in a patient using an ultrasound transmitter allocated to the catheter is disclosed in U.S. Pat. No. 4,697,595 and in the technical note "Ultrasonically marked catheter, a method for positive echographic catheter position identification." Breyer et al., Medical and Biological Engineering and Computing. May, 1985, pp. 268-271. Also, U.S. Pat. No. 5,042,486 discloses a catheter which can be located in a patient using non-ionizing fields and superimposing catheter location on a previously obtained radiological image of a blood vessel. There is no discussion in either of these references as to the acquisition of a local information, particularly with electrical activation of the heart, with the locatable catheter tip and of possible superimposition of this local information acquired in this manner with other images, particularly with a heart chamber image.